1. Field of the Invention
This invention relates to target simulators for testing radar and, more specifically, to a low cost, moving target simulator for testing a wide variety of radars without the need for a direct hard wire connection between the simulator and the radar under test.
2. Description of the Related Art
Numerous radar systems have built-in "target simulators" or even "moving target simulators" as test or training target-track pattern generators. Many such systems are directly connected to the radar under test and are therefore incapable of providing independent testing of the entire radar system, including its antenna. The systems usually require placing the radar in a special operating mode such that the radar cannot be used in its normal target search, acquisition and tracking modes.
This special mode does not permit the radar to be simultaneously subjected to the various deleterious signals of the external environment such as either unintentional or intentional electromagnetic interference. Additionally, there is usually no provision in these simulator systems for varying target return in order to evaluate radar system minimum detectable signal or to impose realistic inverse-4th law distance-dependent target return power variation.
Many existing radar target simulators are dedicated to a particular radar system and are incompatible with other radar systems. The existing radar simulators, whether dedicated or non-dedicated, often cost in the hundreds of thousands of dollars. The enhanced cost of these simulators is largely attributable to their complex design.
For example, the so-called DRFM-based (digital radio frequency memory) simulator requires operationally complex and expensive components to enable the capture and delayed retransmission of radar pulses. These components require significant power consumption and thermal dissipation.
In a DRFM-based simulator, the radio frequency waveform i.e. the pre-detection waveform, is sampled, resulting in a large number of samples which must be acquired at very high speed over the duration of a received radar pulse. The sampled waveform is stored in high speed memory, recalled with an appropriate delay, and is then retransmitted.
For example, a radar pulse width of 0.25 .mu.sec requires an RF bandwidth of more than 4 MegaHertz which then requires a DRFM sampling rate of more than 10 MegaHertz. For a typical pulse Doppler radar with a 20 to 1 variation in pulse width, the total number of samples which must be taken in a maximum 5 .mu.sec pulse width will be in excess of 50. Unless the DRFM time-tags each pulse, which increases complexity, it must continuously take samples that are later "played-back" with a delay appropriate for the distance of the simulated target. To simulate a target distance of eighty miles, there must be a delay of 1 millisecond. For such a delay, over 10,000 samples would need to be taken at the sampling rate of 10 MegaHertz.
An ideal target simulator should be versatile enough to accommodate the wide variety of radars currently in service as well as those planned for service in the near future. The radar target simulator should be useable for radar testing, training, logistics, security, and maintenance reasons. The simulator should also be useable remotely as well as directly with a radar system under test. Such a simulator must replicate (and retransmit), in both time and amplitude variation, and with an appropriate delay, an incoming pulse stream from a radar under test.
As the amplitude of the pulses received by a simulator will vary as an azimuthally scanning radar antenna beam sweeps across the simulator's antenna, it is important that the simulator's output track this amplitude variation. Ideally, the target simulator should also be able to create a variety of target trajectories, e.g. constant velocity, a continuous cycling between specified minimum and maximum distances, the ramping up and down in velocity of a target, etc., as well as being able to simulate variable target power such as constant or inverse-4th power of distance. While meeting the above, such a device should be as simple as possible as well as inexpensive.